The question under discussion is more serious than it seems at first glance, and it concerns understanding the essence of a planet.
Once every three years, the International Union of Astronomers holds a general conference attended by thousands of scientists from around the world. The conference is intended, as is customary for such a conference, to serve as a meeting place between researchers from different countries to exchange opinions, keep up-to-date and debate topics that are usually only of interest to astronomers. This year, the Astronomical Conference was held in Prague, and unlike the previous conferences, it received wide media coverage because of the stormy discussion about Pluto. The debate revolves around the question of whether Pluto is a planet, or whether it does not belong to the prestigious club of planets orbiting the Sun - a club that includes, in addition to Earth, the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune.
The question under discussion is more serious than it seems at first glance, and it concerns understanding the essence of a planet. Since Pluto was discovered on February 18, 1930 at the edge of the solar system, at a distance of about 6 billion kilometers from the sun, it has been considered the ninth planet in the planetary system. However, even then it was clear that its orbit was different from the orbits of the eight known planets. The movement of all eight planets is a regular and moderate movement - they move in almost circular orbits, that is, they maintain an almost constant distance from the sun, they are all very close to the same plane - the plane of the Earth's motion, and do not cross each other's orbit. Pluto's orbit is different - it is elliptical, it is tilted relative to the Earth's plane at an angle of 17 degrees, and its orbit intersects Neptune's orbit. In addition, Pluto is much smaller than all the other planets - its mass is estimated to be about a quarter percent of the material found on Earth, for example. Therefore, the doubts about Pluto's belonging to the planet club grew stronger over the years. It should be remembered that the solar system contains many other bodies besides the planets. The asteroid belt, for example, which lies between Mars and Jupiter, contains more than three hundred thousand asteroids with known orbits around the Sun.
Pluto's position at the far end of the solar system and its different orbit can, perhaps, indicate that our planetary system "only" reaches the orbit of Neptune, which is 4.5 billion kilometers from the sun. It is possible that beyond Neptune, which is a giant planet whose mass is 18 times greater than the mass of the Earth, only small bodies are formed in orbits without noticeable regularity. Over the years since the discovery of Pluto, about 800 other small bodies have been discovered that move around the Sun beyond Neptune, and they also have large inclination angles and elliptical orbits. This group has been called the Kuiper Belt in recent years. Opponents of Pluto's association with the group of planets see it as only one of the large bodies in the Kuiper belt. A similar situation exists in the asteroid belt. The largest of the asteroids - Ceres, approaches Pluto in size and mass.
The two belts, the Kuiper belt and the asteroid belt, testify, perhaps, to the process of the formation of the planets. The accepted theory today claims that the two belts represent the situation that prevailed in the solar system 4.5 billion years ago, at a time when small bodies were moving in disorder throughout the system. Some of these bodies stuck to each other and formed one large body - a planet, which managed to reject all the other bodies in its area. In the region between Mars and Jupiter and in the region beyond Neptune, the process of contagion was not successful, and therefore we see there the situation that prevailed in the entire system before the formation of the planets.
The question of the definition of a planet has worsened recently, when in recent years, with the improvement of the means of observation, "new planets" were discovered beyond Neptune, such as "Sedna", for example, whose size approaches the size of Pluto. In 2003, 2003 UB313, sometimes called "Xena", was discovered, surpassing even Pluto in size. The growing number of planets has forced the Astronomical Society to rethink and try to define as precisely as possible what a planet is.
The International Astronomical Society, like any large and unwieldy body, established a committee more than a year ago to discuss the matter. The committee team decided, after hesitation and internal debates, to recommend adopting a definition that would include Pluto as a planet. The committee took into account, of course, the public significance of the decision. The opinion that Pluto is the ninth planet has gained a strong hold among the general public. In developed countries, where basic astronomy is studied in elementary school, almost every child knows the nine planets. In the USA, the children are even taught how to remember the names of the nine planets and their order by memorizing the amusing sentence My Very Educated Mother Just Served Us Nine Pickles whose initials (words) are like the nine planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. Note that the number nine appears by itself within the sentence.
By the way, such a play on words also appears in our sources, in the verse "To the Lord over all deeds" said in the Shabbat morning prayer and its subject is the praise that glorifies God in all His creatures, including the stars. The author compares and says - "Call upon the sun and light will shine; see and establish the shape of the brick; praise is given to him by all the armies of heaven." The initials of the last part of the passage are the initials of the six planets that were known to the author of the Piot - Saturn, Venus, Venus, Mercury, Jupiter, and Mars.
The committee's recommendation was brought to the approval of the plenary of the conference and caused a storm of spirits and strong opposition there. Following this, the committee retracted its position and after a few days brought to the plenum an opposite resolution proposal, removing Pluto from the group of planets. According to the decision that was approved by an overwhelming majority, only a body that has no similar bodies in its vicinity can henceforth be called a planet, and therefore we have only eight planets left. To sweeten the bitter pill of Pluto fans, the committee invented a new classification - dwarf planets, which includes Pluto, 2003 UB313, and the asteroid Ceres.
The decision made in Prague closes a circle in the history of astronomy. At the beginning of the 17th century, the Prague moon was a critical crossroads in the emerging understanding of the movement of the planets. Johannes Kepler, who worked and lived in Prague, published in 1609 the book "New Astronomy" containing revolutionary laws regarding the movement of the planets, laws that are still valid today. Kepler's laws shattered the Greek concept that saw the planets as ideal bodies belonging to the realm where the gods are, and therefore they must move in circles with perfect symmetry and constant speed. It turns out that the planets are not like that, and their movement is closer to the physical earthly movements. In the same year, 1609, Galileo assembled his first telescope which also contributed to a more earthly perception of the moon and planets. At the end of that period, astronomers realized that the same laws of physics apply both to bodies on Earth and to the planets.
If so, Prague is the way station where a new astronomy began that saw the planets as bodies moving in terrestrial orbits that are far from perfect symmetry. Since 2006, Prague is a new stop on the path of the definition of planets. Henceforth, a planet is not any body moving around the sun in a Keplerian orbit. A planet is now just a large body that evolved from the previous state of the solar system and swallowed most of the small bodies that were in that region. The definition of a planet is not based only on its current trajectory but on its past formation trajectory according to our current understanding.
* A shorter version of the article was published yesterday, 5/9/06 in Haaretz newspaper. This version is presented on the website with the permission of the author. The author is a professor and chairman of the Institute of Astronomy at Tel Aviv University
